Rainfall thresholds for triggering a debris avalanching event in the southern Appalachian Mountains
D. G. Neary, L. W. Swift, Jr., 1987. "Rainfall thresholds for triggering a debris avalanching event in the southern Appalachian Mountains", Debris Flows/Avalanches, John E. Costa, Gerald F. Wieczorek
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In early November 1977, a storm system that formed in the Gulf of Mexico moved northeastward into the Appalachian Mountains. It produced intense (as much as 102 mm/hr) and heavy (200-300 mm) rainfall that set off debris avalanching in steep terrain of the Pisgah National Forest, North Carolina. Antecedent rainfall during September and October was 177 percent of normal and the wettest on record for these 2 months. The storm began on 2 November, and rainfall was relatively continuous and even (20-50 mm/day) for the next 3 days. The long-duration rainfall was capped by intense convective downpours the night of 5-6 November when debris avalanching occurred. Peak intensities measured at 15 gauges near Asheville, North Carolina, ranged from 21 to 102 mm/hr, with nearly half exceeding 75 mm/hr. Return intervals for peak intensity rainfall in the range of 75 to 102 mm/hr are 50 to 200+ yr. Total storm rainfall for these gauges ranged from 35 to 250 mm, with peak 24-hr rainfalls of 30 to 180 mm. Rainfall intensities for 1-, 3-, 6-, 12-, and 24-hr periods at a gauge near one avalanching site were 69,137,159,164, and 180 mm, respectively.
Development of the storm was monitored by GOES infrared satellite imagery in real time, and flash flood warnings were issued. Debris avalanching and high stormflow produced peak stream flows with return periods ranging from 20 to 100+ yr. The largest debris avalanches occurred on steep slopes (70% +), started at high elevations (900-1,100 m) in shallow residual soils (less than 1 m deep), had tracks commonly greater than 700 m, and carried a volume of material averaging 2,500 m3 per avalanche.